I just started using FoxyProxy about 3 minutes ago and I already love it.  As somebody that does a lot of security testing through a variety of proxies as well as tunneling my standard HTTP traffic through my home proxy, I know have a solution to avoid mixing the two.  FoxyProxy has the capability to use a proxy based on a regular expression or pattern, so it is now extremely easy to proxy all testing traffic through my HTTP interceptor of choice.